Pregnancy Calculator

From LMP, add 280 days (40 weeks). This is Naegele's rule. From conception date, add 266 days (38 weeks). Both assume a 28-day cycle with ovulation around day 14. Cycles vary, so the date is an estimate, not a guarantee.

Most people know LMP more reliably than the exact day of conception. Pregnancy is dated from LMP by convention, so 'gestational age' is two weeks longer than the actual time since conception. Ultrasound can refine the date, especially in the first trimester.

Gestational age is how far along the pregnancy is, expressed as weeks and days from the first day of the last menstrual period. A typical pregnancy reaches term at 37 to 42 weeks. The calculator computes gestational age from today's date.

First trimester is roughly 0 to 13 weeks, second trimester is 14 to 27 weeks, and third trimester is 28 weeks through delivery. Exact week boundaries vary slightly by source, but those are the common cutoffs.

Longer or shorter cycles shift ovulation away from day 14, which shifts the conception date. For irregular cycles, ultrasound dating is more reliable than calendar dating. A clinician may revise the due date based on the first trimester ultrasound.

Yes. Ultrasound dating is the most accurate single estimate in the first trimester (typically within 5 to 7 days). If the calendar-based estimate differs significantly from the ultrasound, clinicians usually adopt the ultrasound date.

The LMP-based calculation is accurate within about ±1 week for women with regular 28-day cycles. For women with irregular cycles or unknown LMP, accuracy decreases significantly. Early ultrasound dating (between 8-13 weeks) provides the most accurate estimate, typically within 3-5 days. Only about 4-5% of babies are born exactly on their due date — 80% are born within 10 days on either side.

Enter the first day of your last menstrual period in the calculator above. The "Gestational age" output shows your current pregnancy week and day (e.g., 14 weeks 3 days). Pregnancy is typically counted from the first day of your LMP, not from conception — which means by the time most women confirm pregnancy, they're already 4-6 weeks along by this count.

The second trimester begins at week 14 and ends at week 27. The first trimester is weeks 1-13. The third trimester begins at week 28 and runs to delivery (typically week 40). The calculator above shows your current trimester based on today's date and your due date.

It's unlikely but possible, especially for women with short cycles (21-25 days). Sperm can survive in the female reproductive tract for up to 5 days. If you ovulate early — say, day 10 or 11 of a short cycle — having sex during a longer period (days 6-8) could potentially result in pregnancy. For more on fertility windows, see our ovulation calculator.